+----------------------------------------------------------+
| STScI Analysis Newsletter (STAN)
| for the Advanced Camera for Surveys (ACS)
| ACS STAN #4, 3 April 2003
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CONTENTS:
1. Cycle 12 and ACS
2. Dealing with hot pixels, and dithering vs CR-SPLIT
3. A dither pattern for the HRC occulting finger
4. Charge Transfer Efficiency (CTE)
5. Exposure time estimates for red targets in F850LP
6. Reminder on WFC buffer dump overheads
7. Subarrays for WFC and HRC
8. CCD Bias calibration and Gains in Cycle 12
9. ACS Coronagraphy
10. New version of Tiny Tim HST PSF simulation software
11. Pipeline automatically providing flat-fielded images
12. Calibration Workshop Proceedings
13. Recent ACS publications
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1. Cycle 12 and ACS
This edition of the STAN provides information that may be
of particular relevance to proposers about to prepare
Cycle 12 Phase II observing programs. The information
collected here covers developments since the Cycle 12 ACS
Instrument Handbook release in October 2002, with an
emphasis on items relevant for proposal preparation. For
assistance with any of the issues discussed here, please
send email to help@stsci.edu .
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2. Dealing with hot pixels, and dithering vs CR-SPLIT
The hot pixel annealing rate for the HRC has been as
expected (~80%), but for the WFC it has been
significantly lower than expected (~60%). The growth of
unannealed WFC hot pixels is progressing at a rate such
that by the middle of Cycle 12 (January 2004)
they will contaminate as many pixels as do cosmic rays in
a typical (~1000 sec) exposure. For more details on this,
see ACS Instrument Science Report 02-09:
http://www.stsci.edu/hst/acs/documents/isrs/isr0209.pdf
While the standard CR-SPLIT approach allows for cosmic-ray
subtraction, without additional dithering it will not
eliminate hot pixels in post-observation processing. Hence,
we recommend that observers who would have otherwise
used a simple CR-SPLIT now use some form of dithering
instead. For example, a simple ACS-WFC-DITHER-LINE pattern
has been developed, based on integer pixel offsets, which
shifts the image by 2 pixels in X and 2 in Y along the
direction that minimizes the effects of scale variation
across the detector. The specific parameter values for
this pattern are given in Section 8.4.3 of the Phase II
Proposal Instructions at http://www.stsci.edu/public/p2pi.html
(This link is no longer available.)
However, any form of dithering providing a displacement of
at least a few pixels can be used to simultaneously
remove the effects of cosmic ray hits and hot pixels
in post-observation processing. The recent development
of PyDrizzle and Multidrizzle in the IRAF/STSDAS
environment now makes this task easy. There are many
resources available to guide users in the reduction of
dithered data, including cosmic ray rejection and hot
pixel elimination. See for instance:
http://www.stsci.edu/hst/acs/analysis/drizzle
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3. A dither pattern for the HRC occulting finger
Some HRC users have been surprised to see a substantial
region masked out by the occulting finger in their data.
The occulting finger is not retractable - it will be
in every HRC exposure. However, as with any other detector
feature or artifact, the "lost" data can be recovered by
combining exposures which were suitably shifted with
respect to each other. The following link provides more
information on the HRC occulting finger, and a specific
dither pattern to allow recovery of data lost behind
the finger:
http://www.stsci.edu/hst/acs/faqs/finger.html
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4. Charge Transfer Efficiency (CTE)
The first external calibration data taken on ACS to measure
the effects of CTE degradation on stellar photometry are
currently being analyzed. Preliminary results match
expectations. For typical observing backgrounds and stellar
fluxes, CTE losses on WFC are typically 1-2% far from the
readout amplifier (parallel read-out) and readily
correctable. Only in the worst case scenarios (i.e., faint
sources observed with narrow-band filters) do CTE losses
reach ~10%-20%. Observers requiring better-than-1% precision
should consider mitigating CTE by placing sources near the
read-out amplifier and giving careful consideration to
backgrounds. No measurable CTE effects are seen for the
HRC at this time.
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5. Exposure time estimates for red targets in F850LP
At wavelengths greater than 7500 A (HRC) and about 9000 A
(WFC) ACS CCD observations are affected by a red halo due
to light scattered off the CCD substrate. An increasing
fraction of the light as a function of wavelength is
scattered from the center of the PSF into the wings. For
the very broad z-band, F850LP, filter in particular the
encircled energy depends on the underlying spectral energy
distribution; such an effect is not yet incorporated in
the available (CGI) exposure time calculator (ETC).
In order to obtain correct estimated count rates for red
targets observers are advised to use the Synphot package
in IRAF/STSDAS for which a proper integration over
wavelength has now been incorporated for encircled energy.
To quantify this new synphot capability, we compare ETC
results with synphot for a set of different spectral
energy distributions and the observation mode
WFC,F850LP. In the following table, the spectral type
is listed in the first column. The fraction of light with
respect to the total integrated to infinity is listed in
the other two columns, for the ETC and synphot calculations
respectively. These values are derived for a 7x7 pixel box
for the ETC calculations and for a 0.2 arcsec aperture
for synphot:
Sp. Type ETC Synphot
O 0.76 0.74
M 0.76 0.7
L 0.76 0.68
T 0.76 0.6
The ETC results are off by 3% (O star), 9% (M star),
12% (L star), and 27% (T star). If this small effect is
relevant to particular observations, then the Synphot
software package can be used. A new keyword has been
implemented to call for the encircled energy tables.
The keyword is "aper". The user is allowed to select an
aperture (radius in arcsec) and indicate this value by
typing "aper#value". Currently, the following apertures
are supported: every tenth of arcsec between
0. and .6 arcsec, 0.8, 1., 1.5, 2. and 4. arcsec.
When calling "aper#0." the user will obtain the number of
counts in the brightest pixel, i.e. the peak counts of the
source centered at that pixel.
Arbitrary aperture sizes are also permitted but are not
recommended, because Synphot provides only a linear
interpolation between supported apertures, which is a poor
approximation, especially at small apertures. A typical
obsmode would now read like: acs,wfc1,aper#0.2,f850lp.
From the command line such an obsmode should be entered
within quotes, or synphot complains:
cl> calcphot "acs,wfc1,aper#0.2,f850lp"
GOs can always "epar" the program they want to run, or use
Pyraf instead of cl. Users can also provide their own input
spectrum, which has to be in the same format as the user's
provided spectrum for the ETC. Please refer to the ETC help
pages for more specific information about this format. The
file should be placed in the directory synphot is run from.
+----------------------------------------------------------+
6. Reminder on WFC buffer dump overheads
Users of the WFC should be reminded that it takes a long
time (nearly six minutes) to read out and transfer ACS/WFC
data for each exposure. For WFC exposures 338 seconds is a
magic number; for longer exposure times the next readout
and data buffer dump will be hidden in the next exposure;
for shorter exposure times the telescope and ACS will sit
idle for an extra six minutes after the exposure to deal
with the data. This is discussed well in the ACS Handbook
(Chapter 9), but thought worth pointing out here. Somewhat
new for Cycle 12 is improved support of subarrays which can
further help ameliorate buffer management overheads, see
the next item for details on new subarrays.
+----------------------------------------------------------+
7. Subarrays for WFC and HRC
In cycle 12, selection of general subarrays will be
available, and in some cases, supported. In this context,
supported means that bias frame calibrations will be
supplied by STScI for a predefined set of subarrays. See:
http://www.stsci.edu/hst/acs/faqs/subarrays.html
+----------------------------------------------------------+
8. CCD Bias calibration and Gains in Cycle 12
The ACS Instrument Handbook for Cycle 12 included
details on readout noise and full well depths for the two
CCD instruments (e.g. see Chapter 7). The supported GAINS
in Cycle 12 are HRC = 2, 4 electrons/DN; WFC = 1,2. The
Cycle 11 calibrations were concentrated for the default
GAINS (WFC 1, HRC 2). Experience has shown that calibration
reference files are independent of gain selected with the
important exception of biases which have unique
gain-dependent structure. We will provide equally good
calibration reference files at both of the supported GAINs
for HRC and WFC in Cycle 12. Users should therefore feel
free to choose the best gain for their science independent
of calibration program considerations.
+----------------------------------------------------------+
9. ACS Coronagraphy
The new developments for this may be found in Instrument
Science Report ACS 2002-11, "ACS Coronagraph Update for
Cycle 12 Proposers," by John Krist:
http://www.stsci.edu/hst/acs/documents/isrs/isr0211.pdf
+----------------------------------------------------------+
10. New version of Tiny Tim HST PSF simulation software
Version 6.1 of Tiny Tim will be released in mid-April 2003.
This will be the first update since ACS was put into orbit
and includes significant enhancements to its ACS PSF
modeling capabilities. On-orbit aberrations are included as
well as the most recent geometric distortion coefficients.
Field-dependent aberrations have also been updated.
Field-dependent CCD charge diffusion is now also included,
for both the WFC and HRC (this is the largest factor in the
variation of the PSF FWHM over the field of view). Tiny
Tim is available at:
http://www.stsci.edu/software/tinytim
+----------------------------------------------------------+
11. Pipeline automatically providing flat-fielded images
As of 12 March 2003, the ACS calibration pipeline (CALACS)
has begun automatically and always providing the individual
flat-fielded images (*flt.fits) which were used to produce
the combined flat-fielded products, such as the cosmic-ray
cleaned (*crj.fits) and drizzled (*drz.fits) images.
Previously, these files were only provided for some types
of observations, so users who desired them had to set
EXPSCORR=PERFORM and re-run CALACS themselves. Since these
individual flat-fielded images are the input for
MultiDrizzle, they have become essential for the reduction
of most ACS observations, regardless of how the exposures
were defined and executed. For information on MultiDrizzle,
please see the STSDAS Web page.
+----------------------------------------------------------+
12. Calibration Workshop Proceedings
Proceedings from the October 2002 HST Calibration Workshop
are now available online:
http://www.stsci.edu/hst/HST_overview/documents/calworkshop/workshop2002/Archived webcasts of the presentations are also
available for viewing.
+----------------------------------------------------------+
13. Recent ACS publications
This listing was generated with STEPsheet:
http://stepsheet.stsci.edu/
ANDERSON, J. "Astrometry with the Advanced Camera:
PSFs and Distortion in the WFC and HRC"
2002 HST Calibration Wksp. 13-22, 2003
BOND, H.E.; HENDEN, A.; LEVAY, Z.G.; PANAGIA, N.;
SPARKS, W.B.; STARRFIELD, S.; WAGNER, R.M.; CORRADI, R.L.M.;
MUNARI, U. "An Energetic Stellar Outburst Accompanied by
Circumstellar Light Echoes" Nature 422: 405-408, 2003
CASERTANO, S.; GOODS TEAM; LAIDLER, V.; CAPAK, P.;
COWIE, L.; HUBBLE HIGHER-Z SN TEAM; HICKEN, M.
"Supernovae 2002kh, 2002ki, and 2003N" IAUC 8052: 1, 2003
CLAMPIN, M.; SIRIANNI, M.; BLAKESLEE, J.P.; GILLILAND, R.L.
"Status of the Advanced Camera for Surveys"
2002 HST Calibration Wksp. 3-12, 2003
COX, C.; GILLILAND, R.L. "The Effect of Velocity
Aberration on ACS Image Processing"
2002 HST Calibration Wksp. 58-60, 2003
DAHLEN, T. "Supernovae 2003az, 2003ba, 2003bb, 2003bc,
2003bd, 2003be" IAUC 8081: 1, 2003
DE GRIJS, R.; LEE, J.T.; MORA HERRERA, M.C.;
FRITZE-V. ALVENSLEBEN, U.; ANDERS, P.
"Stellar Populations and Star Cluster Formation in
Interactivng Galaxies with the Advanced Camera for Surveys"
NewA 8: 155-171, 2003
FAN, X.; STRAUSS, M.A.; SCHNEIDER, D.P.; BECKER, R.H.;
WHITE, R.L.; HAIMAN, Z.; GREGG, M.; PENTERICCI, L.;
GREBEL, E.K.; NARAYANAN, V.K.; LOH, Y.-S.; RICHARDS, G.T.;
GUNN, J.E.; LUPTON, R.H.; KNAPP, G.R.; IVEZIC, Z.;
BRANDT, W.N.; COLLINGE, M.; HAO, L.; HARBECK, D.; PRADA, F.;
SCHAYE, J.; STRATEVA, I.; ZAKAMSKA, N.; ANDERSON, S.;
BRINKMANN, J.; BAHCALL, N.A.; LAMB, D.Q.; OKAMURA, S.;
SZALAY, A.; YORK, D.G. "A Survey of z > 5.7 Quasars in the
Sloan Digital Sky Survey. II. Discovery of Three Additional
Quasars at z > 6" AJ 125: 1649-1659, 2003
GILLILAND, R.L.; RIESS, A. "Extreme Red Sensitivity of
ACS/WFC" 2002 HST Calibration Wksp. 61-64, 2003
MACK, J.; BOHLIN, R.C.; GILLILAND, R.L.; VAN DER MAREL, R.;
DE MARCHI, G.; BLAKESLEE, J.P. "ACS Flat Fields and
Low-Order 'L-flat' Corrections from Observations of
47 Tucanae" 2002 HST Calibration Wksp. 23-30, 2003
MEURER, G.R.; LINDLER, D.; BLAKESLEE, J.P.; COX, C.;
MARTEL, A.R.; TRAN, H.D.; BOUWENS, R.J.; FORD, H.C.;
CLAMPIN, M.; HARTIG, G.F.; SIRIANNI, M.; DE MARCHI, G.
"Calibration of Geometric Distortion in the
ACS Detectors" 2002 HST Calibration Wksp. 65-69, 2003
MUTCHLER, M.; KOEKEMOER, A.M.; HACK, W.
"Drizzling Dithered ACS Images - A Demonstration"
2002 HST Calibration Wksp. 70-73, 2003
PASQUALI, A.; PIRZKAL, N.; WALSH, J.R.
"The Wavelength Calibration of the WFC Grism"
2002 HST Calibration Wksp. 38-46, 2003
PIRZKAL, N.; PASQUALI, A.; WALSH, J.R. "Flat-fielding of
ACS WFC Grism Data" 2002 HST Calibration Wksp. 74-77, 2003
RATNATUNGA, K.U. "Statistical Analysis of ACS Data without
Covariance in Errors" 2002 HST Calibration Wksp. 78-81, 2003
RIESS, A. "Growth of Hot Pixels and Degradation of CTE
for ACS" 2002 HST Calibration Wksp. 47-52, 2003
SIRIANNI, M.; DE MARCHI, G.; GILLILAND, R.L.; BOHLIN, R.C.;
PAVLOVSKY, C.; MACK, J. "On-Orbit Sensitivity of ACS"
2002 HST Calibration Wksp. 31-37, 2003
SIRIANNI, M.; MARTEL, A.R.; JEE, M.J.; VAN ORSOW, D.;
SPARKS, W.B. "Bias Subtraction and Correction of
ACS/WFC Frames" 2002 HST Calibration Wksp. 82-85, 2003
SPARKS, W.B. "ACS Calibration Software"
2002 HST Calibration Wksp. 53-57, 2003
TRAN, H.D.; MEURER, G.R.; FORD, H.C.; MARTEL, A.R.;
SIRIANNI, M.; BOHLIN, R.C.; CLAMPIN, M.; COX, C.;
DE MARCHI, G.; HARTIG, G.; KIMBLE, R.; ARGABRIGHT, V.
"On-Orbit Performance of the ACS Solar Blind Channel"
2002 HST Calibration Wksp. 86-89, 2003
TRAN, H.D.; SIRIANNI, M.; FORD, H.C.; ILLINGWORTH, G.D.;
CLAMPIN, M.; HARTIG, G.; BECKER, R.H.; WHITE, R.L.;
BARTKO, F.; BENITEZ, N.; BLAKESLEE, J.P.; BOUWENS, R.;
BROADHURST, T.J.; BROWN, R.; BURROWS, C.; CHENG, E.;
CROSS, N.; FELDMAN, P.D.; FRANX, M.; GOLIMOWSKI, D.A.;
GRONWALL, C.; INFANTE, L.; KIMBLE, R.A.; KRIST, J.;
LESSER, M.; MAGEE, D.; MARTEL, A.R.; MCCANN, W.J.;
MEURER, G.R.; MILEY, G.; POSTMAN, M.; ROSATI, P.;
SPARKS, W.B.; TSVETANOV, Z. "Advanced Camera for Surveys
Observations of Young Star Clusters in the Interacting
Galaxy UGC 10214" ApJ 585: 750-755, 2003
WALSH, J.R.; PIRZKAL, N.; PASQUALI, A.
"Modelling the Fringing of the ACS CCD Detectors"
2002 HST Calibration Wksp. 90-93, 2003
YAN, H.; WINDHORST, R.A.; COHEN, S.H. "Searching for z~=6
Objects with the Hubble Space Telescope Advanced Camera
for Surveys: Preliminary Analysis of a Deep Parallel Field"
ApJ 585: L93-L96, 2003
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